The present invention relates to an ignition apparatus for an internal combustion engine, and more particularly, to such an ignition apparatus of the capacitor discharge ignition (CDI) type.
FIG. 3 shows a typical example of such an ignition apparatus of the capacitor discharge ignition type for an internal combustion engine. In this figure, a DC power source 1 in the form of a battery for use with a motor vehicle is connected to a DC-DC converter or amplifier 2 which serves to convert or amplify the output voltage of the battery 1 to several hundred volts. A rectifying diode 3 has an anode coupled to an output terminal of the converter 2 and a cathode coupled to one end of a capacitor 4, the other end of which is connected to an ignition coil 5. The ignition coil 5 has a primary winding and a secondary winding which is connected to a spark plug 6. An ignition signal generator 7 is connected to the battery 1 at a node between the battery 1 and the converter 2, and it generates an ignition signal in synchronism with the rotation of an internal combustion engine (not shown). A trigger circuit 8 is connected to the battery 1 and the ignition signal generator 7 for generating a trigger signal in response to an ignition signal from the ignition signal generator 7. The ignition signal generator 7 and the trigger circuit 8 together constitute a signal generating means. A switching element 9 in the form of a thyristor has an anode connected to a node between the rectifying diode 3 and the capacitor 4 and a cathode connected to the primary winding of the ignition coil 5 and to ground. The thyristor 9 has a control gate connected to an output terminal of the trigger circuit 8 so that it is made conductive or switched on by a trigger signal from the trigger circuit 8 to thereby allow the capacitor 4, which is charged by the battery 1 through the DC-DC converter 2 and the rectifying diode 3, to discharge by way of the primary winding of the ignition coil 5. The DC-DC converter 2, the diode 3, the capacitor 4, the ignition signal generator 7, the trigger circuit 8 and the switching element 9 are housed in a power unit 10, as depicted by a phantom line in FIG. 3, whereas the ignition coil 5 is housed in a casing 11 disposed separately from the power unit 10, the ignition coil 5 being connected to the elements in the power unit 10 by means of a wire harness 12 which includes a conductor wire 12a connected between the capacitor 4 and the ignition coil 5 and a conductor wire 12b connected between the primary winding of the ignition coil 5 and the switching element 9.
In operation, the DC-DC converter 2 increases or amplifies the DC voltage generated by the battery 1 to an enlarged voltage of several hundred volts, which is then supplied via the rectifying diode 3 to the capacitor 4 for charging it. When the switching element 9 is made conductive or turned on by a trigger signal from the trigger circuit 8, which is generated in response to an ignition signal produced by the ignition signal generator 7 at a predetermined crankshaft position or ignition timing, the charged capacitor 4 begins to discharge by way of a closed discharge path comprising the capacitor 4, the switching element 9, the conductor wire 12b, the primary winding of the ignition coil 5, the conductor wire 12a and the capacitor 4. As a result, a high voltage is developed across the secondary winding of the ignition coil 5, thus causing the spark plug 6 to generate a spark.
With the known ignition apparatus as described above, upon discharge of the charged capacitor 4, a large discharge current flows through the discharge path including the now conductive switching element 9, the wire harness 12 and the ignition coil 5. In this connection, however, since the ignition coil 5 alone is formed separately from, and connected through the wire harness 12 to, the other components such as the capacitor 4 and the switching element 9 included in the discharge path, the discharge path becomes long. That is, the use of the wire harness 12 connecting the ignition coil 5 with the capacitor 4 and the switching element 9 inevitably results in a long or elongated discharge path, which not only forms a major noise source but also leads to a relatively large loss in electrical energy.